Interferometers are optical instruments used for precisely measuring displacements by reflecting a laser beam off a target and analyzing the optical characteristics of the returning beam. In a commonly used heterodyne interferometer, a laser beam having orthogonally polarized components of different frequencies is used. After splitting the components with a polarizing beam splitter, one component is reflected off the movable target and the other is reflected off a fixed reference. The components are recombined and the resulting beat frequency is analyzed. Because each component carries a distinct signal, it is important to avoid "crosstalk" between the orthogonally polarized components.
In such conventional interferometers, some crosstalk occurs because the beam components are not precisely orthogonal to each other. Typical nonorthogonality values may range up to several degrees. This generates nonlinearity errors that skew measurement results. Such errors have been small enough to be acceptable in many applications, but are problematic in critical applications such as microlithography.
Existing systems use additional optics and receiver electronics to reduce the nonorthogonality typical of most systems, but these systems are often bulky and of limited effect. Because of the thermal and turbulence effects that reduce measurement precision, existing beam sources may be positioned outside the target environment, but this requires an optically precise window, which ay generate errors. This also adds to the machine footprint.
The present invention overcomes the limitations of the prior art by generating a beam with substantially orthogonally polarized components, splitting the beam to separate the components to separate paths, projecting each component into a respective polarization preserving optical fiber (PPF), recombining the components output from the fibers into a single beam, and transmitting the single beam to an interferometer. The PPFs may be rotationally adjusted at their output ends to provide precise orthogonality in the resulting beam.